The present invention relates to a picture transmission system that transmits digitized pictures coded by inter-frame coding.
Systems that transmit digitized moving pictures over communication networks, including videophone systems, videoconferencing systems, and video-on-demand (VOD) systems, are becoming increasingly common, and work is proceeding on the international standardization of methods of coding the moving pictures. Coding methods can be classified as intra-frame methods, which reduce spatial redundancy within an individual picture or frame, and inter-frame methods, which also reduce temporal redundancy from frame to frame. One inter-frame coding method is described in recommendation H.263 of the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). Another is described in the phase-two standard of the Moving Picture Experts Group (MPEG-2), which has been adopted by the International Standards Organization (ISO). An example of a system that uses only intra-frame coding is a system using the still-picture coding method of the Joint Photographic Experts Group (JPEG). JPEG coding, which is also an ISO standard, can be used to code each frame of a moving picture, creating what is called a JPEG movie.
Inter-frame coding is much more efficient than intra-frame coding. On a typical transmission channel with limited bandwidth, inter-frame coding can provide smoother motion and higher picture quality than can intra-frame coding, as long as no transmission errors occur. When a transmission error occurs, however, inter-frame coding causes the effect of the error to propagate from frame to frame, which can lead to persistent and serious picture degradation.
Transmission errors are, unfortunately, quite common in communication systems. They are particularly common in wireless communication systems. They also occur in packet transmission networks, such as local area networks employing the asynchronous transmission mode (ATM), which can lose data due to packet collisions or overflow of buffers.
A simple method of dealing with transmission errors is to retransmit the erroneous or missing data, but this method is generally too slow for moving-picture transmission systems, which must operate in real time.
Another method is to employ a powerful error-correcting code, with sufficient redundancy to make most errors correctable at the receiving end. The redundant bits use up bandwidth, however, so the picture data must be compressed to a greater degree in compensation. The higher compression ratio entails a sacrifice of picture quality, even when no transmission errors occur. Furthermore, no error-correcting code can correct all transmission errors. Severe burst errors and errors caused by packet loss generally remain uncorrectable.
For these reasons, systems employing inter-frame coding usually refresh the picture by inserting frames coded by, intra-frame coding (intra-frames) from time to time. Intra-frames may be inserted at regular intervals, a practice known as cyclic refresh, or in response to signals from the receiving terminal, a practice known as refresh on demand.
A problem is that since intra-frame coding is less efficient, a certain amount of picture quality must be sacrificed each time an intra-frame is transmitted. If intra-frames are inserted frequently, the attendant loss of picture quality can become quite noticeable, the extreme case being a JPEG movie. If intra-frames are transmitted infrequently, however, there may be an undesirably long delay before picture degradation caused by a transmission error is corrected.
For reference, FIG. 1 shows a simple example of a moving picture transmitted with inter-frame and intra-frame coding. Frames 1 and 10 (hatched) are intra-frames, also referred to as I-frames. The other frames 2, 3, 4, 5, 6, 7, 8, 9, 11, 12 are inter-frames, also referred to as predicted frames or P-frames. Each P-frame is coded with reference to the immediately preceding frame, as indicated by the arrows.
FIG. 2 illustrates the effect of a transmission error in frame 6 of this series of frames. Frame 6 cannot be decoded correctly, because of the error; frame 7 cannot be decoded correctly, because it was coded with reference to frame 6; frame 8 cannot be decoded correctly, because it was coded with reference to frame 7; and frame 9 cannot be decoded correctly, because it was coded with reference to frame 8. The result of this error propagation is that the picture is degraded in four consecutive frames, the correct picture not being recovered until the next intra-frame is transmitted, at frame 10.
FIG. 3 shows a JPEG movie, in which every frame is coded by intra-frame coding. Transmission errors do not propagate from frame to frame, but the inefficiency of JPEG coding precludes the transmission of a high-quality picture at a smooth-motion frame rate within practical bandwidth limits.
An object of the present invention is to recover from picture transmission errors quickly, with minimal loss of picture quality.
In a picture transmission system that transmits a series of pictures from a coding apparatus to a decoding apparatus, each picture in the series being coded and decoded with reference to a preceding picture in the series, the invention provides a transmission-error recovery method comprising the following steps:
detecting a decoding error caused by the transmission error;
transmitting an error signal from the decoding apparatus to the coding apparatus;
selecting a reference picture;
determining a part of the reference picture made undecodable by the decoding error;
modifying this part of the reference picture in the coding apparatus;
identically modifying the same part of the same reference picture in the decoding apparatus;
coding a subsequent picture with reference to the modified reference picture; and
decoding the subsequent picture with reference to the modified reference picture.
The invented method is quick, because it does not wait for intra-frame coding to take place.
The reference picture is modified as little as necessary, so high coding efficiency is maintained. Consequently, high picture quality is maintained.
The invention also provides a coding apparatus and a decoding apparatus employing the invented method.